If your purchasing this just for WDW and its pricey I would think twice. There is a very good chance that when you get to the gates it will not be allowed. WDW is pretty strict about Wagons - and this does seem like a Wagon. I have been entering a theme park when a family has tried to enter with a wagon and it was not allowed. Even if a CM tells you its ok - that's no guarantee it will be allowed (unfortunately).
Here is the quote from the WDW website:
Are there any personal items I should not bring with me into the Theme Parks?
A. Items that you may not bring into the Theme Parks include, but are not limited to:
Items with wheels,
such as wagons, skateboards, scooters, inline skates, shoes with built-in wheels, two-wheeled or three-wheeled conveyances, strollers larger than 36" x 52", suitcases, coolers, or backpacks with or without wheels larger than 24" long x 15" wide x 18" high (coolers required for medication may be stored in a locker or at Guest Relations), and any trailer-like object that is pushed or towed by an
ECV wheelchair or stroller
Alcoholic beverages
Weapons of any kind
Folding chairs
Glass containers (excluding baby food jars and perfume bottles)
Pets (unless they are service animals)
In Disney's Animal Kingdom® Theme Park (for the safety of the wildlife), balloons, straws and drink lids are not permitted.
Here is the link so you can verify - its the last question:
http://disneyworld.disney.go.com/wdw/common/helpFAQ?id=HelpFAQThemeParkPage#q13
Personally, in your situation I would just budget a few $$ and rent its so easy, really. My dd's are 2 years apart as well and the price of renting by the time they were 4and 6 was well worth the convenience. Since we didn't need it at the resort there was no need to lug it on/off the bus tram boat. We would usually start the day without one and were able to hit all of one area of a park esp fantasyland without the hassle of parking and re-parking etc. By the end of the day if one or both were tired we would rent especially for an evening at Epcot or a long day at AK.
The rental strollers are very easy to push even with older kids, they are easy for the kids to hop in and out of and if it gets wet just wipe it down or you can trade it in for a dry one

. If you do the length of stay you get a discount and skip the hassle of filling out the form and paying every day.
Again - I don't want to discourage you but at your kids ages its alot of $ to spend on a wagon/stroller you may not be able to use. It does look pretty interesting though.
HTH ~
TJ